Super high frequency wattmeter



Oct. 1948. Y EVANS I 2,451,724

SUPER HIGH FREQUENCY WATTMETER Original Filed Aug. '31, 1943 INVENTOR.

AIYWRNEY Patented Oct. 19, 1948 SUPER HIGH FREQUENCY WATTMETER John Evans, Hightstown, N. 3., assignor to Radio Corporation of America, a corporation of Delaware Original application August 31, 1943, Serial No.

500,605, now Patent No. 2,427,094, dated September' 9, 1947. Divided and this application December .6, 1945, Serial No. 633,201

t l This application is a division of my copendin U; S. application, Serial No. 500,605, filed August 31, 1943, now Patent No. 2,427,094, issued September 9, 1947, assigned to the same assignee as the instant application.

This invention refers generally to super-highfrequency measuring apparatus and more particular'ly to wattmeters for measuring super-highfrequency energy in a wave guide transmission system.

Heretofore, various methods have been devised for measuring super-high-frequency power transmitted by wave guides. Many of these systems have involved measurements of standing wave transmission system, generating heat in response blown capillary tube portions.

to said absorbed energy, operating indicating means in response to said generated heat, and calibrating said indicating means. The calibration may be made by an auxiliary heat generating device actuated-by a calibrated power source.

A preferred embodiment of the invention comprises a short section of wave guide adapted to be connected to a wave guide transmission system. The wave guide transmission section includes two oppositely disposed apertures in the widest parallel faces of the wave guide section. A capillary U tube, having enlarged blown portions in each end thereof, includes two similar resistive elements, one of which is disposed in each of the One of the resistive elements is disposed within the wave guide section by inserting one end of the U tube in the oppositely disposed apertures whereby the resistive element acts .as an eflicient antenna for energy transmitted by the wave guide. Preferably the value of the resistive element is of the order of the surge impedance of the wave guide transmission system. The remaining resistive element is connected through suitable power measuring instruments to a calibrated power source.

Heatwill be generated in the first-resistive ele- 2 Claims. (Cl. 171-95) 2 ment disposed within the firstwave guide section by currents induced therein by the super-highfrequency field in the wave guide. The resultant heat will expand the gas in the capillary tube and tend to displace a mercury column therein an amount proportional to the heat generated in the resistive element. The calibrated power-or voltage source connected to the second resistor is then adjusted until heat generated by the second resistor neutralizes the displacement of the mercury column caused by the heat generated by the first resistor. The power absorbed by the first resstor from the wave guide section may then be determined directly from the amount of power,

indicated by the power indicating apparatus, re-

quired to neutralize this effect.

Among the objects of the invention is to provide an improved method of and means for measuring super-high-frequency energy. Another object of the invention is to provide an improved method of and means for measuring super-highfrequency energy transmit-ted by a wave guide transmission system. A further object of the invention is to provide an improved method of and means for measuring power in a super-high-frequency wave guide transmission system by means of indicating apparatus responsive to heat generated in a resistive element inserted within the wave guide for absorbing energy therefrom. A

further object of the invention is to provide an improved method of and means for measuring power in a super-high-frequency wave guide transmission system wherein power absorbed in the form of heat by the measuring apparatus provides indications which may be calibrated by ex,- ternally adjustable power measuring means. The invention will be further described by reference to the accompanying drawing of which the single figure is a schematic diagram of a preferred embodiment thereof.

Referring to the drawing, a. wave guide section I having oppositely disposed apertures 2, 3 in the wide parallel faces 4, 5 thereof, includes a flanged portion 6 adapted for engagement with a similar flanged portion of a conventional rectangular wave guide transmission system, A U-shaped capillary tube 7 includes a first hollow blown portion 8 terminating one end thereof, an-d'a second hollow blown portion 9 terminating the remaining end thereof. The capillary portion of the U tube 1, intermediate the blown portions 8, 9, include a mercury column l0.

The first blown portion 8 of the capillary tube I encloses a resistive element It having a resistance of the order of the surge impedance of the wave guide section. The secondblown portion 8 of theU tube I encloses a second resistive element I2, which is connected through a. current indicating meter I3 and a variable resistor II to a source or calibrating voltage, not shown. A voltage indicating meter I5 is connected across the current indicating meter I3 and the second resistor l2. Instead of indicating separately the voltage and current, the indicator may be a wattmeter.

The first resistive element II preferably has a length substantially equivalent to one-halt wavelength at the operating frequency of the wave guide transmission system; whereby the element functions as an antenna for absorbing energy from the wave guide when it is inserted through the apertures 2, 3 into the interior of the guide.

Heat generated in the first resistive element H, by means of the high frequency energy absorbed from the wave guide, provides expansion of the gases in the end of the u tube terminated by the first blown portion 8 thereby tending to displace the mercury column Ill. The calibrating power applied to the second resistive element l2 may be varied by the variable resistor Hi to provide suitable heating '0! the second resistor I2 for neutralizing the displacement of the mercury column Ill. The product of the current and voltage indicated by the current indicating meter I3 and the voltage indicating meter I5 will, therefore, be a measure of the power absorbed from the first resistor II from the wave guide transmis-' sion system. The first resistive element ii is substantially resonant due to its physical dimensions, and has an impedance substantially equal to the surge impedance of the wave guide transmission system, hence, substantially all of th energyavailable in the wave guide transmission system is absorbed by the'resistive element.

Therefore, the power indicated by the product of the readings on the indicating meters 83 and 15 will provide a substantially accurate indication of the total power transmitted by the wave guide.

It should be understood that the particular mechanical, construction of the embodiment of the invention described is purely illustrative and that there may be various modifications thereof within the spirit and scope of the invention. It should further be understood thatthe system may be employed for measuring the power of continuous, damped, interrupted or pulsed wave energy, providing the modulation characteristics are taken into consideration in calibrating theelectrical power applied to the auxiliary compensat-' ing or calibrating resistive element.

I claim as my invention:

1. Apparatus for measuring microwave energy in a waveguide transmission system comprising a first resistive element for translating said microwave energy into heat energy, capillary tube means including a first enlarged portion enclosing said element and extending within said waveguide system and including an indicatingliquid column having its position determined by gas pressure responsive to said heat energy, a second resistive element enclosed within a second enlarged portionof said tube external of said waveguide system, an adjustable source of electrical energy for energizing said second resistive element to oppose changes in position oi. said liquid column by gas pressure responsive to the heat generated in said second element, and callbrated means for adjusting said source to provide comparative indications by the position of said liquid column 01 the energy applied to each of said resistive elements.

2. Apparatus for measuring microwave energy in a waveguide transmission system comprising a first-resistive element substantially matched to the surge impedance of said waveguide system for translating said microwave energy into heat energy, capillary tube means including a first enlarged portion enclosin said element and extending within said waveguide system and including an indicating liquid column having its position determined by gas pressure responsive to said heat energy, a second resistive element enclosed within a second enlarged portion of said tube external of said waveguide system, an adjustable source of electrical energy for energizing said second resistive element to oppose changes in position of said liquid column 'by gas pressure responsive to the heat generated in said second element, and calibrated means for adjusting said source to provide comparative indications by the position of said liquid column of the energy applied to each of said resistive elements.

JOHN EVANS.

REFERENCES crrnn The following references are of record in the file of this patent:

UNI-TED STATES PATENTS Okress May 21, 1946 

